<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Flex Panels 💪</title>
  <link href='https://fonts.googleapis.com/css?family=Amatic+SC' rel='stylesheet' type='text/css'>
</head>
<body>
  <style>
    html {
      box-sizing: border-box;
      background: #ffc600;
      font-family: 'helvetica neue';
      font-size: 20px;
      font-weight: 200;
    }
    
    body {
      margin: 0;
    }
    
    *, *:before, *:after {
      box-sizing: inherit;
    }

    .panels {
      min-height: 100vh;
      overflow: hidden;
      /* -------- */
      display: flex;
    }

    .panel {
     
      background: #6B0F9C;
      box-shadow: inset 0 0 0 5px rgba(255,255,255,0.1);
      color: white;
      text-align: center;
      align-items: center;
      /* Safari transitionend event.propertyName === flex */
      /* Chrome + FF transitionend event.propertyName === flex-grow */
      transition:
        font-size 0.7s cubic-bezier(0.61,-0.19, 0.7,-0.11),
        flex 0.7s cubic-bezier(0.61,-0.19, 0.7,-0.11),
        background 0.2s;
      font-size: 20px;
      background-size: cover;
      background-position: center;
      /* ------- */
      flex: 1;
      display: flex;
      /* 规定flex项目的方向 row 水平 column 垂直*/
      flex-direction: column;
      /* 主轴方向上的对齐方式 */
      justify-content: center;
     
    }

    .panel1 { background-image:url(https://source.unsplash.com/gYl-UtwNg_I/1500x1500); }
    .panel2 { background-image:url(https://source.unsplash.com/rFKUFzjPYiQ/1500x1500); }
    .panel3 { background-image:url(https://images.unsplash.com/photo-1465188162913-8fb5709d6d57?ixlib=rb-0.3.5&q=80&fm=jpg&crop=faces&cs=tinysrgb&w=1500&h=1500&fit=crop&s=967e8a713a4e395260793fc8c802901d); }
    .panel4 { background-image:url(https://source.unsplash.com/ITjiVXcwVng/1500x1500); }
    .panel5 { background-image:url(https://source.unsplash.com/3MNzGlQM7qs/1500x1500); }

    /* Flex Children */
    .panel > * {
      margin: 0;
      width: 100%;
      transition: transform 0.5s;
    }
    /* ------ */
    .panel >*:first-child{
      transform: translateY(-100%);
    }
    .panel >*:last-child{
      transform: translateY(100%);
    }
    .panel.open-active>*:first-child{
      transform: translateY(0%);
    }
    .panel.open-active>*:last-child{
      transform: translateY(0%);
    }
    /* ------- */

    .panel p {
      text-transform: uppercase;
      font-family: 'Amatic SC', cursive;
      text-shadow: 0 0 4px rgba(0, 0, 0, 0.72), 0 0 14px rgba(0, 0, 0, 0.45);
      font-size: 2em;
      /* ------ */
      flex: 1;
      display: flex;
      justify-content: center;
      align-items: center;
    }
    
    .panel p:nth-child(2) {
      font-size: 4em;
    }

    .panel.open {
      font-size: 40px;
      flex:5
    }

  </style>


  <div class="panels">
    <div class="panel panel1">
      <p>Hey</p>
      <p>Let's</p>
      <p>Dance</p>
    </div>
    <div class="panel panel2">
      <p>Give</p>
      <p>Take</p>
      <p>Receive</p>
    </div>
    <div class="panel panel3">
      <p>Experience</p>
      <p>It</p>
      <p>Today</p>
    </div>
    <div class="panel panel4">
      <p>Give</p>
      <p>All</p>
      <p>You can</p>
    </div>
    <div class="panel panel5">
      <p>Life</p>
      <p>In</p>
      <p>Motion</p>
    </div>
  </div>

  <script>
    ;(function(){
      // 获得的不是数组，只可以使用forEach和.length
      const panels = document.querySelectorAll('.panel');
      function clickHandle(){
        // console.log(this);
        // 如果open类名存在，则删除它，如果不存在则添加它
        this.classList.toggle('open');
      }
      // 当动画结束后，p变化
      function transitionHandle(e){
        // e.propertyName 获取变化的属性名
        // 在这里获取到的属性名，会因为浏览器的不同，有不同的值
        // console.log(e.propertyName);
        // 抓取flex的变化，触发事件
        if (e.propertyName.includes('flex')) {
        this.classList.toggle('open-active');
      }
      }
      panels.forEach(panel => {
        panel.addEventListener('click',clickHandle);
        // transitionend事件会多次执行，可能会导致最后的动画效果抵消，所以在事件中抓取一个关键的 去触发事件
        panel.addEventListener('transitionend',transitionHandle);
      })
    })();
  </script>



</body>
</html>
